BEIRUT Syria's opposition on Saturday said it had agreed to the "possibility" of a temporary truce, provided there were guarantees Damascus's allies including Russia would cease fire, sieges were lifted and aid deliveries allowed country-wide.

Russian air strikes began last September and turned the tide in Syria's five-year civil war in Bashar al-Assad's favour, to the frustration of the United States and its allies who have supported rebels trying to topple the president.

Attempts to negotiate a truce in recent months have failed. The latest round of talks at the United Nations in Geneva is being jointly chaired by Russia and the United States.

On Saturday, various Syrian opposition factions "expressed agreement on the possibility of reaching a temporary truce deal, to be reached through international mediation," a statement from the High Negotiations Committee said.

A halt to Russian bombardments did not look immediately likely.

The spokesman for Russia's President Vladimir Putin, Dmitry Peskov, said: "Russia is sticking to its consistent policy of rendering assistance and aid to the armed forces of Syria in their offensive actions against terrorists and against terrorist organizations."

The High Negotiations Committee, which joins various armed groups and Syria's exiled political opposition, said the U.N. must guarantee "holding Russia and Iran and sectarian militias ... to a halt to fighting". All sides should cease fire simultaneously and the government should release prisoners, it added.

U.S. and Russian military officials met ahead of the wider Geneva meeting, diplomats said on Friday.

A source close to peace talks earlier on Saturday told Reuters Syria's opposition had agreed to a two- to three-week truce.

The truce would be renewable and supported by all parties except Islamic State, the source said. It would also be conditional on the al Qaeda-linked Nusra Front no longer being targeted, at least to start with, the source said.

The Nusra Front is considered a terrorist organisation by the U.N. Security Council and banned.

Asked if the opposition's insistence on the Nusra Front no longer being targeted was the main stumbling block, he described it as "the elephant in the room".

"They have to deal with this very delicately or they are going to end up with a civil war in Idlib on their hands," the source said.

Nusra fighters are fighting alongside other rebel groups in some areas, including Idlib.

Ending sieges on civilians have become another key sticking point in talks to end the conflict.

The U.N. estimates there are 486,700 people in around 15 besieged areas of Syria, and 4.6 million in hard-to-reach areas. In some, starvation deaths and severe malnutrition have been reported.

(Reporting by Tom Perry and John Davison in Beirut; Additional reporting by Tom Miles in Geneva and Dmitry Solovyov in Moscow; Editing by Janet Lawrence and Raissa Kasolowsky)

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